These techniques will be most successful when attempting to introduce new species into an open sod. They are much less effective on dense grass sods.
One non-chemical strategy for controlling weeds prior to seeding a pasture is to till or mow every 14 to 21 days throughout the summer, then follow with a late summer seeding in early to mid August. This helps deplete the weeds carbohydrate reserves, weakening and eventually killing them. Late summer seeding means fewer annual weed problems as well. Do not use a companion crop or graze these seedings until the following spring.
There are a number of herbicides registered for use in pasture situations. Not all are intended for use on newly seeded pastures. It is important to read product labels carefully. Producers should pay particular attention to pasture species registered for application, stage of pasture development registered for application, weeds species controlled, and grazing intervals. A complete list of products registered for use in pastures and hayfields and the common weeds controlled in these situations are listed in Table 17. of the Guide to Crop Protection 2004 Page 43.
Many pastures become severely infested with thistles and other problem weeds due to continuous grazing. Cattle will over-graze areas of young, succulent growth and under-graze more mature areas. Overgrazing results in an open sod that allows light to penetrate to weed seeds and seedlings. Under grazing can be harmful as well, as excessive growth will smother new shoots, inhibit tiller development and weaken the desirable species. This will create open spots which allow weed encroachment.
Once pastures are established, it is important to keep weeds from invading and reducing pasture condition.
The time to begin your weed control strategy in a pasture is in the establishment phase. If weeds are not controlled at the outset, they may choke out a new seeding or allow weeds to encroach into the established stand.
A properly managed rotational grazing system avoids these problems. Cattle are left in a paddock until the grass has been grazed to the proper height, then are moved to another paddock and so on. The original paddock is grazed again only when it has had sufficient time to recover. This helps maintain a healthy, vigorous pasture which can easily compete with weeds. The proper rest period for a paddock depends on the species and time of year. Rest periods of 15 to 20 days are common in early spring. In late summer, when growth has slowed, rest periods may be as long as 35 days. Maintaining proper fertility and pH is also critical in maintaining a healthy stand and decreasing weed encroachment.
Proper grazing management has proven to be a very useful weed control mechanism. Good fertility goes hand in hand with good weed control. Controlled grazing allows beneficial plants to become strong, productive plants and out-compete the weeds. Rotational grazing helps in this process because it gives beneficial plants the opportunity to rest after grazing, and then grow undisturbed before being grazed again. High stocking rates in small paddocks can also be used to “force” animals to consume forage that might normally be considered a “weed.”
Plants such as Canadian thistles are difficult to control due to their ability to produce a second seed head during the same growing season. This forces a follow-up clipping. In addition to re-growing, they commonly produce the second seed head close to the ground, making clipping very difficult. Caution must also be observed when mowing woody plants, as some plant material such as cherry tree leaves can become toxic to livestock after they have been artificially desiccated.
Weed control in grass pastures is therefore limited to controlling broadleaf weeds and is generally accomplished with post-emergence, translocated herbicides. These herbicides are absorbed by the foliage and move within the plant. As a result, they may produce a toxic effect a considerable distance from the point of entry. Post-emergence applications are greatly affected by the age of the weeds and the growing conditions. Applications should be made when the weeds are young and actively growing. Weather conditions are also important consideration as the herbicide may need time to be absorbed.
Good weed control takes dedication and utilization of several methods, especially with certain weeds. Specific options include: 1) grazing management, 2) mechanical control, and 3) chemical control.
Chemical control of weeds is often looked at as the first option of control, but consideration should go into developing a plan. Chemical weed control can be a challenge because of its selective or nonselective means of controlling specific plant species. For example, you can use 2-4,D to control thistles in some cases. The problem is that 2-4,D will also kill desirable legumes that naturally persist or that were planted. It is essential to read chemical labels and also use resource people such as your local UMaine Extension county educator.